A write head converts a current signal carrying digital information into a magnetic field. This magnetic field impresses a flux pattern on a magnetic tape as the tape passes the write head. A read head then senses the recorded flux pattern to recover the digital signal. One common input write signal is shown in FIG. 1a. Binary signal 20 is converted to input write signal 22. Input write signal 22 is a non-return-to-zero inverted (NRZI) signal. In this particular NRZI code, each one is represented by a data transition, one of which is indicated by 24, and each zero is indicated by the lack of a transition as related to a data clock in receiver electronics. When input write signal 22 is fed to a write head, and tape 26 is moved over the write head, data fields 28,30 are written onto tape 26 as shown in FIG. 2a. Each data transition 24 causes a change in magnetization direction between adjacent data fields 28,30.
When tape 26 is passed over a read head, data fields 28,30 are converted to read output signal 32. Electronics connected to the read head use means such as a threshold detector to recover binary signal 20 from read output signal 32. However, as can be seen in FIG. 3a, a long string of zeros in binary signal 20 causes a large swing in read output signal 32. This complicates the read electronics.
One way of considering the problem is that the long string of zeros in binary signal 20 results in long data field 30 on tape 26. Flux field 30 is a magnet. The greater the length of data field 30, the greater the strength of the resulting magnet. Therefore, reducing the large swings in amplitude of read output signal 32 can be achieved by breaking up long data field 30.
A method for breaking up long data field 30 is to include short pulses at high frequency in input write signal 22. This produces a signal known as write-equalized input signal 34 shown in FIG. 1b. Equalization pulse 36 is added to input write signal 22 at locations representing some or all of the zeros in binary signal 20. Equalization pulse 36 consists of a signal outside the effective frequency range of the read head and channel. When write-equalized signal 34 is written onto tape 38, as shown in FIG. 2b, equalization pulse 36 is written as high frequency field 40. This may be likened to high frequency erasure as the high frequency recording is not reproduced by the read head.
When tape 38 including high frequency fields 40 is read by the read head, each high frequency field 40 is sensed as a region producing no flux density. Read output signal 42, shown in FIG. 3b, therefore does not include the large amplitude swings seen in read output signal 32 from tape 26 not having high frequency fields 40. Hence, simpler thresholding circuitry may be used in read electronics.
Many techniques are possible for determining where to place equalization pulses 36 in write-equalized input signal 34. For example, each zero in binary signal 20 can generate a corresponding equalization pulse 36. The technique for generating the pattern shown in FIG. 1b together with additional techniques for generating write-equalized input signal 34 and a discussion of write equalization is included in "Write Equalization For Generalized (d,k) Codes" by Richard C. Schneider, IEEE TRANSACTIONS ON MAGNETICS, Vol. 24, No. 6, November 1988, pp. 2533-2535, which is hereby incorporated by reference.
A cross-sectional view of a prior tape head for writing write-equalized input signal 34 onto tape 38 is shown in FIG. 4. Write head 50 includes bottom pole 52, top pole 54, and conductive coil 56 which together form an electromagnet. Referring to FIGS. 2b and 4, operation of write head 50 will be described. When current is applied to conductive coil 56, a magnetic field is produced across the gap, shown generally by 58. As tape 38 moves past gap 58, fields 28,30,40 are written onto tape 38.
One difficulty with prior tape head 50 is that the amplitude of input current required to produce a desired magnitude of magnetic field at gap 58 for equalization pulse 36 is much greater than the amplitude of current required to produce a magnetic field at gap 58 that has substantially the same magnitude for data transition 24. This results in complicated write equalization circuitry to produce write-equalized input signal 34.
Other difficulties arise if sufficient equalization cannot be added when tape 38 is written. First, complicated read equalization circuitry is required to reshape detected data transitions 24. This reshaping may require boosting high frequency components which may degrade the read signal-to-noise ratio. Second, the lack of sufficient equalization causes larger swings in the magnetization seen by the read head. These larger swings increase distortion due to nonlinearities in the read head. Third, record depth is greater than necessary since low frequency signals record at greater depth on tape 38 than high frequency signals. Increased record depth may result in degraded overwrite of tape 38 and limited range on the velocity of tape 38 over head 50.
What is needed is a thin film write head that does not require substantially greater input current magnitude for equalization pulse 36 than for data transition 24 to produce substantially equal magnetic field strength amplitude in gap 58. This tape head should be economical to produce and should be similar in construction to prior tape heads.